MDEV-27042 UCA: Resetting contractions to ignorable does not work well

The weight scanner routine scanner_next() did not properly handle the cases
when a contraction produces no weights (is ignorable).

Adding a helper routine my_uca_scanner_set_weight() and using
it in all cases:

- A single ASCII character
- A contraction starting with an ASCII character
- A multi-byte character
- A contraction starting with a multi-byte character

Also adding two other helper routines:

- my_uca_scanner_next_expansion_weight()
- my_uca_scanner_set_weight_outside_maxchar()

to avoid using scanner->wbeg directly inside scanner_next().
This reduces the probability of similar future bugs.

mysql-test/std_data/ldml/Index.xml

@@ -9,6 +9,9 @@
           <i>\u0029</i> <!-- right parenthesis -->
           <i>\u002B</i> <!-- plus -->
           <i>\u002D</i> <!-- hyphen -->
+          <i>tel.</i>
+          <i>tél.</i>
+          <i>ťel.</i>
       </rules>
     </collation>
     <collation name="utf8mb3_test_ci" id="353">